1. Field of the Invention
The present invention pertains to a power supply apparatus that includes a fuel cell and a capacitor.
2. Description of the Related Art
A fuel cell is a possible power supply device for an electric vehicle. Drive power for the vehicle can be obtained by supplying the electric power generated by the fuel cell to the drive motor of the electric vehicle. JP 09-298806A discloses a power supply apparatus that includes a capacitor as well as a fuel cell. A capacitor is a charge accumulator that has a much higher power density than a typical secondary battery, and also has a high charging and discharging efficiency. Therefore, when a power supply apparatus with a capacitor is used, the energy efficiency of the entire system can be improved by efficiently collecting the energy obtained via motor regeneration during braking of the electric vehicle.
The output voltage of the capacitor is directly related to the amount of charge remaining therein, and the output voltage falls as the amount of remaining charge decreases. As a result, where regeneration is performed by the motor when there is little remaining charge in the capacitor, the generated electric power is quickly absorbed, and the voltage in the capacitor increases as the amount of remaining charge increases. When the motor subsequently consumes electric power, the capacitor in which the voltage increased as described above supplies electric power to the motor before the fuel cell. In this way, the amount of remaining charge in the capacitor decreases together with the voltage, and the capacitor becomes ready for the next regeneration.
Incidentally, a fuel cell system has the characteristic that its energy efficiency decreases significantly during low power output periods due to the output characteristics of the fuel cell. In other words, auxiliary devices such as pumps pertaining to the supply of fuel and air consume a predetermined amount of electric power when the fuel cell is operating, but as the amount of electricity generated drops, the percentage of electric power consumed by the auxiliary devices increases relative to the total power generation. Consequently, the energy efficiency of the fuel cell system as a whole declines during low power output periods. Accordingly, the power supply apparatus with a capacitor is desired in which not only is the electric power obtained from regeneration used efficiently, thereby improving energy efficiency, but in addition, adequate energy efficiency of the system as a whole is maintained even during low power output periods during which the efficiency of the fuel cell system declines.